So I thought this would be in Phobos, basically I want to reverse the "polarity" of a value in a range.

What I mean is if I have a possible range of 0 .. 127, then the values of the left would equal the inverted values on the right:

0 -> 127
10 -> 117
20 -> 107
50 -> 77
77 -> 50

and so on.. Quite easy to do, is it common enough to be in Phobos though?